1. Field of the Invention
The present invention relates to an apparatus and method, for use in a satellite-based communications network, for minimizing blocking of communication between the network and access terminals resulting from differences in the signal propagation delays for the access terminals due to their different locations within a coverage area serviced by a spot beam generated by a satellite in the network. More particularly, the present invention relates to an apparatus and method for minimizing such communication blocking by segregating the coverage area of the spot beam into at least one coverage zone, segregating the communication carriers available for the spot beam into a number of carrier groups corresponding to the number of coverage zones, and assigning to each carrier group a specific burst offset time period in accordance with which communication bursts are transmitted over carriers in the carrier group between the network and access terminals located within the coverage zone serviced by the carrier group.
2. Description of the Related Art
A satellite communications network, such as a geosynchronous earth orbit mobile communications network, comprises at least one geosynchronous earth orbit satellite, a ground-based advanced operations center (AOC) and spacecraft operations center (SOC) associated with the satellite, at least one ground-based gateway station (GS), and at least one access terminal (AT), which is typically a hand-held or vehicle mounted mobile telephone. The satellite enables the access terminal to communicate with other access terminals, or with other telephones in the terrestrial public switched telephone network (PSTN), via the gateway stations under the control of the gateway stations. The AOC provides system-wide resource management and control functions for its respective satellite, and the SOC controls on-orbit satellite operations for its respective satellite.
To communicate with access terminals, the network controls a satellite to generate at least one spot beam, which is typically an L-band frequency signal, toward the surface of the earth. Each spot beam covers a predetermined geographic region of the earth, thus enabling access terminals within that region to communicate with the network via communications signals transmitted to the satellite over a carrier selected from a plurality of carriers assigned to the spot beam.
For example, when an access terminal places a call to another access terminal or to a terrestrial telephone, the access terminal generates and transmits a channel request message on a random access channel (RACH) at a frequency assigned to the spot beam to the satellite. Typically, a channel request message includes data representing a random number, which is used as an identifier for the access terminal sending the channel request message, as well as contention resolution and timing synchronization information.
The satellite includes a receiver which, under the control of the network, establishes time frames of a particular duration during which channel request messages are received. When a channel request message is received by the satellite during a particular time frame, and is thus received by the network, the network transmits data via the satellite to the access terminal to establish a communication link between the access terminal and network. The data includes access channel information (access grant channel information) indicating the frequencies of the carriers over which communication between the network and the access terminal is to occur during the call. Typically, a carrier of a particular frequency is assigned to service transmission of communications from the satellite to the access terminal, and a carrier of another frequency is assigned to service transmission of communications from the access terminal to the satellite. Hence, a pair of carriers service communication between the satellite and an access terminal.
Communication between the network and access terminal occurs in the form of signal bursts of a predetermined duration which are transmitted over the carrier pair designated by the access grant channel information between the access terminal and satellite. Signal bursts transmitted from the satellite to the access terminal, along with signal bursts being transmitted to other access terminals, if any, also assigned to a carrier in the carrier pair, are transmitted over the carrier in a time-division multiple access (TDMA) manner. That is, each signal burst being transmitted from the satellite to the access terminal is time-multiplexed with the signal bursts being transmitted by the satellite to the other access terminals in a TDMA frame of a particular duration, and transmitted over the carrier.
A TDMA frame includes a plurality of timeslots, which become occupied by the time-multiplexed signal bursts being transmitted. For example, a TDMA frame can include 24 timeslots, and each signal burst can be 3 timeslots long. Accordingly, a 24 timeslot TDMA frame can contain up to eight signal bursts which are being transmitted to eight respective access terminals (i.e., 8 signal bursts of 3 timeslots each), with each burst occupying three specific sequential timeslots of the TDMA frame. Naturally, a 24 timeslot TDMA frame can accommodate only four signal bursts which are each 6 timeslots in length, with each signal burst occupying six specific sequential timeslots of the TDMA frame.
Upon receiving its appropriate signal burst transmitted from the satellite, each access terminal transmits a signal burst back to the satellite in a TDMA frame over the other carrier in the carrier pair. An access terminal begins transmitting its respective signal burst at an appropriate instant in time after the instant in time at which the access terminal began receiving its respective signal burst transmitted from the satellite as described above. Because the transmitter/receiver of an access terminal is a typically a half-diplexer which permits only signal transmission or signal reception at any given time, the time period in which the access terminal transmits a signal burst can not overlap the time period in which the access terminal is receiving a signal burst.
Specifically, each access terminal begins transmitting its respective signal burst at an appropriate time after receiving a signal burst, which will properly position the signal burst in the TDMA frame so as not to overlap any other signal burst being transmitted by another access terminal, and so that the signal burst will be received at the satellite at the appropriate receive time. An access terminal determines when transmission of the signal burst should begin based on the amount of burst offset required at the satellite, which is the desired duration of time which should elapse from the start of transmission by the satellite of a signal burst to the access terminal and the subsequent start of receipt by the satellite of a signal burst transmitted from that access terminal. The access terminal also takes into account an estimated time which will elapse between the instant when a signal burst is transmitted by the access terminal and the instant when that signal burst is received by the satellite, which is known as the propagation delay.
An access terminal estimates the propagation delay for a distance measured from its location on the earth's surface to the satellite, which is orbiting at about 22,000 miles above the earth's surface. Because the distance from the surface of the earth at the equator to the satellite is less than the distance from the surface of the earth in the extreme northern and southern hemispheres to the satellite, the propagation delay for a signal burst sent from an access terminal close to the equator is less than that for a signal burst sent from an access terminal in, for example, northern Europe. An access terminal can include global positioning system (GPS) equipment, or any other suitable device, which enables it to determine its location on the earth's surface, and thus estimate its propagation delay based on an estimated distance between the location of the access terminal and the satellite.
If the propagation delays for all access terminals being serviced by a particular spot beam were equal, each access terminal would be able set its signal burst transmitting time based on the burst offset for the satellite to assure that the transmitted signal burst would be positioned properly in a TDMA frame and reach the satellite during the appropriate time period. However, because the access terminals are typically at different locations within the spot beam and accordingly, their distances to the satellite differ, they experience different propagation delays.
Therefore, although an access terminal at one edge of the spot beam may be able to base its signal burst transmission timing on the satellite burst offset and experience proper signal burst transmission, an access terminal at the opposite edge of the spot beam may need to base its signal burst transmission timing on a different burst offset value to compensate for the different propagation delay time. In this event, the network may need to control the satellite to set different burst offsets for communication with the access terminals having different propagation delays. Also, movement of an access terminal within the spot beam during communication can alter the propagation delay for that access terminal by an amount large enough to require a corresponding change in burst offset for communication with that access terminal.
As stated above, the use of different burst offsets for communication with access terminals at different locations within the spot beam generally will be sufficient to assure that the signal bursts transmitted from the satellite to the access terminals will reach the access terminals during the appropriate receiving times and vice-versa However, if the access terminals base their respective signal burst transmission timing on different burst offsets, the signal bursts transmitted may not be properly positioned within the TDMA frame to utilize the TDMA frame most efficiently.
That is, the transmitted signal bursts may be distributed in the timeslots of the TDMA time frame such that unused time slots are present between adjacent transmitted signal bursts. In this event, the TDMA time frame is unable to accommodate the maximum amount of time bursts (e.g., eight 3-timeslot long bursts). Accordingly, a transmitted signal burst from one or more access terminals, which would normally be capable of fitting in the TDMA time frame if the transmission bursts were arranged efficiently, is unable to be transmitted in the IDMA time frame over the carrier. This phenomenon is known as "call blocking", in which an access terminal is prevented from transmitting a signal burst in a TDMA time frame over a particular selected carrier, and thus must use a different carrier for transmission. As can be appreciated, the call blocking phenomenon reduces the amount of access terminals that can utilize a particular carrier for communication with the network. Furthermore, because a spot beam is assigned with a finite number of carriers, the overall amount of access terminals that can be serviced by that spot beam is consequentially reduced.
Accordingly, a need exists for a system which minimizes the occurrence of call blocking in a satellite-based communications network by providing efficient multiplexing of signal bursts in the TDMA time frames being transmitted between a satellite and access terminals over carriers assigned to the satellite-generated spot beam servicing the access terminals.